介绍对称加密算法，最常用的莫过于DES数据加密算法

DES
DES-Data Encryption Standard,即数据加密算法。是IBM公司于1975年研究成功并公开发表的。DES算法的入口参数有三个:Key、Data、Mode。其中Key为8个字节共64位,是DES算法的工作密钥;Data也为8个字节64位,是要被加密或被解密的数据;Mode为DES的工作方式,有两种:加密或解密。
　　DES算法把64位的明文输入块变为64位的密文输出块,它所使用的密钥也是64位。



通过java代码实现如下：Coder类见 Java加密技术（一）

import java.security.Key;
import java.security.SecureRandom;

import javax.crypto.Cipher;
import javax.crypto.KeyGenerator;
import javax.crypto.SecretKey;
import javax.crypto.SecretKeyFactory;
import javax.crypto.spec.DESKeySpec;


/**
 * DES安全编码组件
 *
 * <pre>
 * 支持 DES、DESede(TripleDES,就是3DES)、AES、Blowfish、RC2、RC4(ARCFOUR)
 * DES                  key size must be equal to 56
 * DESede(TripleDES)    key size must be equal to 112 or 168
 * AES                  key size must be equal to 128, 192 or 256,but 192 and 256 bits may not be available
 * Blowfish             key size must be multiple of 8, and can only range from 32 to 448 (inclusive)
 * RC2                  key size must be between 40 and 1024 bits
 * RC4(ARCFOUR)         key size must be between 40 and 1024 bits
 * 具体内容 需要关注 JDK Document http://.../docs/technotes/guides/security/SunProviders.html
 * </pre>
 *
 * @author 梁栋
 * @version 1.0
 * @since 1.0
 */
public abstract class DESCoder extends Coder {
    /**
     * ALGORITHM 算法 <br>
     * 可替换为以下任意一种算法，同时key值的size相应改变。
     *
     * <pre>
     * DES                  key size must be equal to 56
     * DESede(TripleDES)    key size must be equal to 112 or 168
     * AES                  key size must be equal to 128, 192 or 256,but 192 and 256 bits may not be available
     * Blowfish             key size must be multiple of 8, and can only range from 32 to 448 (inclusive)
     * RC2                  key size must be between 40 and 1024 bits
     * RC4(ARCFOUR)         key size must be between 40 and 1024 bits
     * </pre>
     *
     * 在Key toKey(byte[] key)方法中使用下述代码
     * <code>SecretKey secretKey = new SecretKeySpec(key, ALGORITHM);</code> 替换
     * <code>
     * DESKeySpec dks = new DESKeySpec(key);
     * SecretKeyFactory keyFactory = SecretKeyFactory.getInstance(ALGORITHM);
     * SecretKey secretKey = keyFactory.generateSecret(dks);
     * </code>
     */
    public static final String ALGORITHM = "DES";

    /**
     * 转换密钥<br>
     *
     * @param key
     * @return
     * @throws Exception
     */
    private static Key toKey(byte[] key) throws Exception {
        DESKeySpec dks = new DESKeySpec(key);
        SecretKeyFactory keyFactory = SecretKeyFactory.getInstance(ALGORITHM);
        SecretKey secretKey = keyFactory.generateSecret(dks);

        // 当使用其他对称加密算法时，如AES、Blowfish等算法时，用下述代码替换上述三行代码
        // SecretKey secretKey = new SecretKeySpec(key, ALGORITHM);

        return secretKey;
    }

    /**
     * 解密
     *
     * @param data
     * @param key
     * @return
     * @throws Exception
     */
    public static byte[] decrypt(byte[] data, String key) throws Exception {
        Key k = toKey(decryptBASE64(key));

        Cipher cipher = Cipher.getInstance(ALGORITHM);
        cipher.init(Cipher.DECRYPT_MODE, k);

        return cipher.doFinal(data);
    }

    /**
     * 加密
     *
     * @param data
     * @param key
     * @return
     * @throws Exception
     */
    public static byte[] encrypt(byte[] data, String key) throws Exception {
        Key k = toKey(decryptBASE64(key));
        Cipher cipher = Cipher.getInstance(ALGORITHM);
        cipher.init(Cipher.ENCRYPT_MODE, k);

        return cipher.doFinal(data);
    }

    /**
     * 生成密钥
     *
     * @return
     * @throws Exception
     */
    public static String initKey() throws Exception {
        return initKey(null);
    }

    /**
     * 生成密钥
     *
     * @param seed
     * @return
     * @throws Exception
     */
    public static String initKey(String seed) throws Exception {
        SecureRandom secureRandom = null;

        if (seed != null) {
            secureRandom = new SecureRandom(decryptBASE64(seed));
        } else {
            secureRandom = new SecureRandom();
        }

        KeyGenerator kg = KeyGenerator.getInstance(ALGORITHM);
        kg.init(secureRandom);

        SecretKey secretKey = kg.generateKey();

        return encryptBASE64(secretKey.getEncoded());
    }
}

延续上一个类的实现，我们通过MD5以及SHA对字符串加密生成密钥，这是比较常见的密钥生成方式。
再给出一个测试类：

import static org.junit.Assert.*;


import org.junit.Test;

/**
 *
 * @author 梁栋
 * @version 1.0
 * @since 1.0
 */
public class DESCoderTest {

    @Test
    public void test() throws Exception {
        String inputStr = "DES";
        String key = DESCoder.initKey();
        System.err.println("原文:	" + inputStr);

        System.err.println("密钥:	" + key);

        byte[] inputData = inputStr.getBytes();
        inputData = DESCoder.encrypt(inputData, key);

        System.err.println("加密后:	" + DESCoder.encryptBASE64(inputData));

        byte[] outputData = DESCoder.decrypt(inputData, key);
        String outputStr = new String(outputData);

        System.err.println("解密后:	" + outputStr);

        assertEquals(inputStr, outputStr);
    }
}

得到的输出内容如下：

原文: DES
密钥: f3wEtRrV6q0=

加密后:    C6qe9oNIzRY=

解密后:    DES

    由控制台得到的输出，我们能够比对加密、解密后结果一致。这是一种简单的加密解密方式，只有一个密钥。
    其实DES有很多同胞兄弟，如DESede(TripleDES)、AES、Blowfish、RC2、RC4(ARCFOUR)。这里就不过多阐述了，大同小异，只要换掉ALGORITHM换成对应的值，同时做一个代码替换SecretKey secretKey = new SecretKeySpec(key, ALGORITHM);就可以了，此外就是密钥长度不同了。